FPGA family selection (novice) problem.

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Hello,

I'm new to the FPGA devices (up to now I have done only a few simple project on the CPLDs). I have to design a bandpass FIR filter which will be working with a 16bit ADC, at 100MHz of sampling frequency. I have two problems:

1. Which device family of FPGA is the most suitable for this type of project?

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Since your design is not that big you should go for Cyclone family of devices. Have a look at the comparison table of different Cyclone family devices. Note the difference in the resources they offer (LEs, multipliers, PLLs, IO pins, memory, configuration options etc.) and then decide which one is "sufficiently good" (i.e. you should not consume 100% of the resources with your design; there should be some room left for expandability) for your design. One good way to check this will be to completely code your design in HDL and synthesize in Quartus II software while targetting different devices. The compilation report so generated at the end of synthesis will give you an idea of the consumption of all kinds of resources. If you discover that Cyclone family of devices doesn't cater for your design, you can check other device families eg. Stratix and plus.

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2. Is it possible to make separate PCB for the ADC and FPGA and to connect them by LVDS interface (I'm making a prototype now and I want to preserve the possibility of easy reconfiguration).

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Yes, it is possible although you would like to avoid this (unless there is any special reason or constraint to do so). It is possible to have both ADC and FPGA on the same PCB as it saves real estate on PCB. Please look into some of the schematics of the development and starters kits at Altera website.

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For what should I take care at PCB designs?I will be grateful for any advice or suggestion.

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PCB design is a completely different aspect and I suggest that you read some good textbooks on the topic and consult vendors solely working in this area.

From what you have described so far, you should be able to satisfy your needs with a Cyclone III part. This would be your least expensive option at the moment. As far as what to take care of ... There are a lot of caveats that might get you in trouble. Too many to cover in a forum post.

Initially, you need to make sure you select a large enough device to fit your logic into. If in doubt, get a larger part than what you think you need.

Aside from that. Read the user guide for the FPGA that you select. Do not try to design a PCB without reading the user guide. As you are connecting the FPGA pins on your schematic, refer to the user guide for the functionality of each pin and how it should be connected. Many forum posts are a result of people failing to read the documentation.

Jake

I agree with Jake that Cyclone III is the best choice for that application. I think that you can go directly with a single PCB which includes both the ADC and the FPGA, without complicating your project with two separate boards.

Be careful about the PCB design recommendations from both the FPGA handbook and the ADC datasheet. Typically you will have to care about the 100MHz clock trace, keep it as short as possible between the two chips.

Another typical issue of these kind of applications is the analogue ground and digital ground separation in order to make the analogue ground insensitive to the switching noise coming from the digital ground. The connection point between the two grounds may be under the ADC if it is the only mixed-signal chip, otherwise the optimal connection point depends on how many mixed-signal ICs are in your design and their location on the board. As previously suggested, PCB design is a complex topic and should be addressed by the aid of a reliable book.

Cheers

OD